Black body background for imaging

BACKGROUND OF THE INVENTION

The present invention is directed to a black body background in the form of a light trap suitable for use in an optical imaging system.

Optical imaging systems are known in which an object, such as a sample which is being evaluated, is illuminated with electromagnetic radiation, and radiation reflected by the sample is detected by an imaging device such as a camera or an optical scanner, as shown in FIG. <b>1</b>. The information gathered in this manner may be digitized and stored for processing in a digital computer. The reflected radiation may be detected in a plurality of spectral bands in a process known as multispectral or hyperspectral imaging. See, for example, U.S. Pat. No. 6,166,373 to Chengye Mao issued Dec. 26, 2000. Numerous such systems are known. By analyzing the spectral information gathered in this manner, it is possible to evaluate physical characteristics of the sample, as well as its composition.

An important consideration in such imaging systems is to assure that the radiation sampled by camera or optical scanner is limited exclusively to that which is reflected by the sample. For this purpose, it is conventional to place the sample in front of a non-reflecting surface, such as a flat black painted surface or a non-reflective black textile material. The idea is to prevent any light that has penetrated or gone around the sample from reflecting off the background and back into the camera or scanner. This allows for the capture of a more pure spectral signature from the sample.

Various arrangements have been proposed for reducing the amount of reflected radiation which emanates from the background surface in imaging and scanning systems such as described above. For example, U.S. Pat. No. 5,144,524 discloses the use of an antireflectance film to prevent scanning light from being scattered by particle contamination on a target surface. U.S. Pat. No. 5,745,293, on the other hand, discloses a light trap for use in optical systems and instruments, in which unwanted light is guided into a light trap in the form of an enclosure containing a conical body which is disposed in the light path. The light is attenuated by reflecting it in all directions from the conical body that facilitates its absorption by the internal walls of the enclosure.

U.S. Pat. No. 6,220,715 discloses a light trap that includes a spherical enclosure, with an opening for receiving a light beam. The interior of the enclosure has a light absorbing surface as well as a polished spheroidal reflector body that is disposed in the path of light that enters through the opening. The reflector disperses the incoming light within the sphere so that it can be absorbed.

The purpose of the present invention is to provide a light trap structure that is simple, inexpensive and easy to manufacture, and is suitable for use as a black body background in an imaging system such as described above.

Another object of the invention is to provide such a light trap structure that efficiently traps spurious radiation incident therein.

These and other objects and advantages are achieved by the light trap structure according to the invention, which includes a hollow sphere, with a hollow tube attached to one side in the form of a chimney. The sphere and chimney are coated (inside and out) with a low reflective coating, such as flat black paint, although any low reflectivity coating can be used. The sample to be imaged is placed on top of the chimney with the imaging system and lights above, aiming down into the chimney. Any light that passes through the sample is “captured” in the sphere section, so that it cannot reflect back to the imaging system.

The light trap according to the invention is extremely simple and effective. Initial tests have indicated a 75% reduction in reflected light, when compared to the same materials and coatings used in traditional setups (i.e., flat surfaces).

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an imaging system in which a sample is illuminated by a radiation source and imaged by a scanner or camera;

FIG. 2 is a schematic depiction of a light trap according to the invention incorporated in an imaging system;

FIGS. 3<i>a-c </i>are respective side, front and top views of an embodiment of the light trap according to the invention;

FIG. 4 shows the construction of a chimney portion of the light trap of FIG. 3, in greater detail; and

FIG. 5 is a schematic depiction of another embodiment of the light trap according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of a conventional optical imaging or scanning arrangement in which a light source (<b>1</b>) irradiates an object of interest or “sample” (<b>2</b>), which is to be evaluated, with electromagnetic radiation, which may be visible light or may be beyond the visible spectrum. Reflected radiation (<b>3</b>) from the sample is collected by the scanner (<b>4</b>), digitized and transmitted to the computer (<b>5</b>) for analysis. In such a system, it is important that light (<b>6</b>) that is incident on the background element (<b>7</b>) not be reflected and captured by the scanner (<b>4</b>), which would result in a degradation of the integrity of the image data collected.

FIG. 2 is a schematic side view of the light trap system according to the invention, together with an imaging system (<b>7</b>) and illumination sources (<b>8</b>). The light trap (<b>9</b>) in this embodiment consists of a hollow sphere (<b>10</b>), with a hollow tube (<b>11</b>) attached to one side in the manner of a chimney. As noted previously, the interior and exterior surfaces of both the sphere and the chimney are coated with a low reflectance coating, such as a coat of flat black paint. (In tests conducted so far, a commercially available flat black paint, such as is marketed under the trade name Krylon™ has been used effectively.)

In use, a sample (<b>2</b>) which is to be imaged is placed on top of the chimney (<b>11</b>), with the imaging system (<b>7</b>) and the illumination sources (<b>8</b>) aimed into the opening of the chimney.

According to one aspect of the invention, the chimney (<b>11</b>) is situated at the peripheral edge of the sphere (<b>10</b>), so that a longitudinal peripheral edge of the chimney (<b>11</b>) joins the sphere (<b>10</b>) approximately tangent to its surface. In this manner, light that enters the opening (<b>12</b>) of the chimney (<b>11</b>) is directed into the body of the sphere (<b>10</b>) where it is attenuated and absorbed by the light absorbent coating on the interior of the sphere. This simple arrangement of the sphere and chimney eliminates the need for elaborate structural elements contained within the interior of the sphere (<b>10</b>), such as a conical element or spheroidal element which is necessary with prior art devices.

According to another feature of the invention, for optimum use, no more than 5% of the total surface area of the sphere should be removed by the opening made by the intersection of the cylindrical chimney (<b>11</b>) with the sphere (<b>10</b>) itself.

The light-trap according to the invention may be made out of any rigid material that can be shaped into a sphere and be made smooth on its interior surface. Suitable materials include wood, plaster or plastics, and the like. Optimally, the light trap can also be made out of a metal, such as aluminum. For ease of providing the interior surface with a non-reflective coating, it may be made in two halves, which can be held together in any suitable manner, including external flange(s).

In one embodiment, the light trap according to the invention was manufactured in two halves using Nylon™, in a process known as selective laser sintering, in which a carbon dioxide laser selectively draws a cross-section of the object on a layer of Nylon™ powder. As the laser draws the cross-section, it selectively “sinters” (heats and fuses) the powder, creating a solid mass that represents one cross-section of the part. The system spreads more powder and sinters layer after layer until the object is complete. Other suitable techniques for manufacturing the sphere and chimney are of course well known to those skilled in the art.

FIGS. 3<i>a</i>, <b>3</b><i>b </i>and <b>3</b><i>c </i>show respective side, front and top views of the light trap according to the invention, including the sphere (<b>10</b>) and chimney (<b>11</b>). As can be seen in each of these views, the opening of the chimney includes an exterior overhang, or lip (<b>11</b><i>a</i>), which is included in order to reduce the amount of light which might otherwise be reflected back out of the chimney due to some reflectivity of the surface immediately inside the chimney opening. This exterior overhang is shown in somewhat greater detail in FIGS. 4<i>a </i>and <b>4</b><i>b. </i>

In another embodiment of the invention, depicted in FIG. 5, an elongated curved tubular member (<b>13</b>) may be substituted for the sphere (<b>10</b>) in the embodiment of FIGS. 2-4. The principle of operation of this embodiment is similar, in that light rays (<b>14</b>) that enter the opening are substantially tangential to the curvature of the tube. Thus, such light rays are guided along the length of the tube where they are attenuated by a light absorbent coating, such as flat black paint or the like.

The light trap according to the invention provides a simple and inexpensive structure for preventing the back scattering of light that enters the opening to the light trap. As described previously, the light trap according to the invention can be manufactured in only two parts, and requires no interior structures whatsoever, other than the application of a light absorbent surface coating. Accordingly, the light trap according to the invention is easily and inexpensively manufactured.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.